Ashina Castle is where the game really opens up in terms of farming opportunities, so grinding out some skill points before then is kinda hard.

The most reliable farming spot before Gyoubu is going to be in the Hirata Estate optional area. From the 2nd idol onwards, there are loads of easy-to-kill bandit enemies that can be stealth killed. Run through this area a few times, making use of drop kills where possible and the axe to make short work of the shield enemies, and you should have a healthy supply of sen, exp, and Black Gunpowder (plus, it'll help hone your kiting skills for some of the larger mob groups later on).

Once you've gotten past Gyoubu and can access Ashina Castle itself, there is one farming spot in particular that is very reliable and will be re-used from now on.

Cut through the castle entrance once you've beaten the Blazing Bull, and get up onto the rooftops on the right-hand side of the castle. Then, make your way to the front of the main keep's rooftops with the nightjar ninja.

You can choose to fight them one by one, or just rush past them to get round the back of the castle and into the open window leading to the Upper Castle Antechamber Idol.

Just on from the idol is a single Ashina Elite Fencer who can be stealth killed from behind very easily. He drops a decent amount of exp and sen, as well as basic upgrade materials and even Divine Confetti. A very useful farm with zero risk. No need to kill any other enemies for this farm, just kill the fencer and run back to the idol to reset. Piece of cake.

Once you have access to Mibu Village, you can farm the villagers very easily. The area from the first idol to the 2nd house on the right as well as the harbour is teeming with villagers that are very easily to kill.

While they don't provide too much experience, they do provide a healthy amount of sen, and can be pretty much endlessly farmed. I recommend using a Mibu Balloon of Wealth here to boost your sen income. It's one of the few places worth using one of these.

I would recommend not going further into the village to continue farming as the underground lurker villagers are a real pain to deal with and can easily jeopardise your farming runs, risking a death.

Once you have collected the Mortal Blade, the White Lotus Flower, and the Shelter Stone for Kuro after beating Genichiro, Ashina Castle comes under attack by the Interior Ministry forces. You'll have to make your way back through the castle again for story purposes, but in doing so, you will unlock the next useful farming spot.

This time, we will be reusing the same farming spot from earlier with the Ashina Elite Fencer, only this time, we will continue on to also kill the purple ninja in the hall after him. By this point in the game, a single fencer doesn't provide a good amount of experience or sen by himself, so the added bonus of the purple ninja (who drops considerably more experience and sen) boosts this farming spot back to relevancy.

Both of these guys also drop useful items as well, including sugars and scrap magnetite. Like before, don't bother farming any other enemies, just stealth kill these two and then reset. Risk free and very efficient.

After beating the Divine Dragon, Ashina Castle comes under full-scale attack and you'll have to fight your way through again. Like before, we'll be returning to our original farming spot in the castle, but the fencer has now been replaced with a purple ninja.

For this farm, we'll just be killing the single purple ninja next to the idol rather than continuing on into the main hall where there are more enemies. The ninjas are fairly tough, so getting the risk-free stealth kill on the one allows for an easy farm compared to taking on 4+ in the main hall.

Each kill will yield around 1000 experience and healthy amount of sen as well as useful crafting materials (in particular, lots of Scrap Magnetite). A great all-round endgame farming location.

In addition to the previous farming spot, the stairs leading up to the castle outside make for the best overall farming spot for Fulminated Mercury, a rare and useful crafting material in the late game.

Several Red Hat Samurai and Ashina Samurai/fencers are battling it out on the steps and won't take much notice of you picking them off for free kills. The Red Hats have a low chance to drop the precious mercury you're after, and can be easily backstabbed as you come in from behind. They also drop a good amount of experience and sen as well.

Be wary, however, as these enemies are quite tough and if they focus on you, you might get swamped and overwhelmed.

For this farm, we'll be heading to the Fountainhead Palace.

Once you've made your way to the Flower Viewing Idol next to the bridge where the Giant Carp first appears, you will have passed 1 Noble just before reaching the idol.

These nobles have a low chance (~5%) to drop 1x Lump of Grave Wax, which is a rare and difficult to acquire crafting material required for many of the late-game prosthetic upgrades.

While there are a few able to be picked up, there are not enough for all of the upgrades, and you will have to farm for them. I recommend popping a Mibu Balloon of Possession for this farm as each reset is very short and you can rack up many kills before the Balloon's effect expires. Just kill the one Noble right before the idol and then reset; there's no need to kill any other ones.

With this farm, I could average approximately 1 Lump of Grave Wax per ~3-5 minutes.